Our long-term objective is to achieve a more complete understanding of the physiological regulation of testicular function. More specifically, we are studying the role of prolactin (PRL) in the pituitary ccntrol of steroidogenesis in the testis. We have shown that PRL can stimulate the biosynthesis of testosterone in rats, mice and hamsters, and data from other laboratoreis suggest that PRL may have a similar effect on the testis in the human as well. We will exmine the effects of PRL on the level of LH receptor in the testis under various experimental conditions. We have established that PRL can modify the steroidogenic response of the testes to LH, and these experiments will elucidate the mechanism of this action of PRL. We have recently obtained some evidence suggesting that PRL may mediate the effects of photoperiod on gonadal function in the hamster. To elucidate this unsuspected action of PRL, we will study testicular function in hamsters kept in a short photoperiod to induce testicular atrophy and then treated with PRL, growth hormone or combinations of PRL and gonadotropins. We will also examine the effects of suppressing the release of pituitary PRL by the administration of ergot alkaloids in hamsters kept in a long photoperiod which normally maintains testicular activity. Finally, we will examine peripheral PRL levels and the effects of treatment with ergot alkaloids in animals undergoing testicular recrudescence. In addition to these experiments, we will continue our work on the effects of treatment with PRL or ergot alkaloids in vivo on the responsiveness of the testis to HCG in vivo. We will also initiate studies on the effects of PRL on the adrenal and on the clearance rate of testosterone. The results of the proposed studies will help to define the role of PRL in the physiological regulation of testicular function including the seasonal changes in gonadal activity.